To start a gas turbine aircraft engine, the pilot must actuate several engine components in a proper sequence, and with proper timing, based on the flight conditions and response of the engine.
For example, in a typical start, the pilot must assure that the compressor has attained the proper speed, in order to deliver sufficient air to the combustor. The compressor can be driven in at least two ways. When the aircraft is on the ground, the compressor is often driven by a starter motor. When the aircraft is airborne, the compressor can be either driven by the starter motor, or windmilled by incoming air. The pilot must decide whether to windmill or use the starter motor.
When the compressor achieves sufficient speed, the pilot must then actuate the igniters and then the fuel valve. During this procedure, the pilot must monitor several engine parameters, such as rotor speeds, igniter operation, amount of fuel flow, and exhaust gas temperature (EGT). The latter, EGT, requires especially close attention, because excessive EGT can cause significant damage.
One cause of excessive EGT is compressor stall, wherein the compressor ceases to pump the required amount of air into the combustor, causing fuel-air ratio to rise, which, in turn, causes EGT to increase. Compressor stalls can develop quite rapidly, and, can be difficult to detect for that reason. Further, they require rapid interdiction to cure.
The starting procedure is complicated by the fact that different ambient conditions cause different starting behavior, and the pilot must take this factor into account. Further, different engines, even of the same general type, have different starting characteristics, and the differences can become greater as each engine ages, or "deteriorates."
This pilot involvement in engine starting increases the pilot's workload. In addition, aircraft frequently travel to remote locations, which have minimal on-site maintenance facilities. A problem in starting can cause expensive and time-consuming delays.